I don't know about you, but i don't want to pay fifty bucks for a pack of ten sheets of film that I might not be able to get tomorrow.

Well, I did (it was only $30 at the time hehe) and it's a lot of fun to shoot, but I'm shifting gears a lot, and getting rid of a lot of my gear to focus on what is most important to me so I just don't need it and won't use it like I used to use it.

I have a PA-145 and PA-45 and getting rid of that one and an extra pack of film, I still have a bunch of the FP-3000B left so I'm holding into the PA-145 for now.

Yes, you can use a Polaroid back on a 4x5 camera, but unless you want to be scrounging for film on eBay, be sure to get the sort of back that accepts 4.25 x 3.375 (image area: 3.75 x 2.875) pack film such as FP-100C or the just-discontinued FP-3000B. The full 4x5 packfilm and sheetfilm that go in the larger 4x5 Polaroid backs are no longer available fresh from anyone.

At the risk of hijacking this thread a bit, how do people reliably frame shots on 4x5 with this smaller-format film? I'm about 2 packs into my 4x5 instant adventure but I frequently mis-frame: how can I add the right lines to my ground glass?

unity: I made a cardboard template to put over my ground glass. Or you could perhaps mark it off with a china marker or a dry-erase marker. I found that for portraits, putting the person's head right in the middle of the frame seemed to work, but your mileage may vary.